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Maybe- but unless you want to just throw money away you still need to know exactly what code it was. As I said above there are multiple codes related to the 02 sensor, and having one (or more) doesn't necessarily mean that the sensor itself is bad. Sometimes a wiring problem or mixture issue can set these codes as well so changing the sensor blindly isn't a guarantee the CEL will go away.

P0420 is the code for catalyst inefficiency. It does NOT indicate a bad 02 sensor. It is triggered by the readings from the rear 02 being out of spec because the catalytic converter is no longer doing its job.
The dealer should replace the cat under the emissions warranty, which is still in effect even at 71K.

I got a flashing CEL a week ago that lasted for about 15 seconds.
I was cruising at about 80 MPH when this happened. Nothing seemed out of the ordinary when the CEL was flashing and it just went away 15 sec later and hasn't returned. I have the TurboXS stage one minus the exhaust for over 10K miles now with no problem. So I'm scratching my head on this one ...

For a few months now while on the freeway it would flash 4 times, then disappear. Recently it came on solid for a day, then disappeared the next day.

Help!!!
Could it be the lightweight flywheel (12.6lbs) causing the misfire readings? Doing a search shows up some WRXs getting misfire readings with lightweight f/w but i haven't heard of any RS having this problem.

I have had three run-in's with my CEL. None of which occured until I added the flywheel, header and exhaust (The cat is still stock).

The first was just a blinking light for a few seconds. A week later if came on steadily as a 302 and 304. A week and a half later it came on steadily and was just a 304. All three of these experiences happened within 3 mins of starting the car.

I have no clue what is causing this. Corporate Subie says that it is the header and also maybe the intake.

Any ideas of how to remedy this? My local Subie garage says that it really isn't all that bad since the car isn't running roughly. They think that the ECU is just too touchy. They also said that the ambient temp might have something to do with it. Personally, I would like to know how to fix it because I get an ulcer when the light is on.

Code: Defintion P0440- EVAP EMMISSION CONTROL SYSTEM MALFUNCTION
Year/Make/Model MY00 Subaru Impreza 2.5RS Coupe
Mileage: 50 Grand
Fix for problem: Was a solid CEL. Gas cap wasn't tightened all the way. The CEL didn't go off immediately after putting gas- in fact, the CEL went off with about 1/10 of the gas left in the tank. Solution was to fill tank with gas, re-tighten the cap, go to the local parts store, borrow the OBDII scanner and confirm it is a P0440 CEL code, go home, take negative battery terminal out for 15mins, press on brake for a few seconds, re-connect battery, and voila! note: the 'erase' function on the OBDII scanner doesn't clear the CEL. You have to re-set the ECU by dissconnecting battery to get rid of the CEL...

I still don't understand why a CEL has to go off with a simple loose gas cap. Why couldn't they make the gas light go off instead? Kinda freaked me out when the "CHECK ENGINE" light goes out in the middle of the highway... Not to mention dealers, Pepboys who wants to charge $40, $79.99 (respectively) to tell you that you have a loose gas cap.

When the gas cap is loose, the evap system test performed by the ECU will fail because the tank cannot be pressurized unless the cap is tight. As a result the code will set. The ECU has no way of knowing that the reason for the leak is a loose gas cap, it could just as well be a leak in a hose or other component. So there is no specific code for "loose gas cap", it's just that a loose cap will create an evap system leak.
If the CEL is indeed due to a loose cap and the cap is then tightened, there is no need for a reset. As long as the condition that caused the CEL to set has been corrected the CEL will go off by itself after three subsequent driving cycles without the problem being detected.
And the scanner should be able to clear the CEL, either they have a very cheap one or they just didn't want to do it for some reason. My scanner will do this and so will most other generic OBDII scanners.

this is the code i hate the most. Well i had this code about 9-12 times. I reset the ecu or clear the codes myself. It just keeps on appearing. It appeared again today.
When this first came on i took it to the mechanic and they said they can't find any problems with the car. Took it them about 3 times whenever the light came on
I took it to the dealer about 3 times and they charge alot to just take a look at it. They have changed the o2 censors and some hose and replace the gas cap.
Well, the car to run fine. No problems with anything, but the lights just comes on once in a while and i just clear the codes.

To: sir_aspen_white
Can you tell me if your CEL comes on again or is it fixed. I want to know how to get rid of this cel. I have reseted the ecu about 3 times before, but it just comes back on and now i just clear the

Well, our CEL has gone off & on about 3 times now over a period of about 3 months. It started about 10K ago. Has gone off for a few days and then come back on. Right now it's been off since Monday (natch, the day I finally called the Dealer to make an appointment...)

As reported in earlier post(s), it's a 2001 2.5 RS, 72k, code is P0420, or was last week when I had it read.

Anyone have a similar experience with the light going out for a few days & then reappearing?

My friend was telling me he printed out a complete list of CE codes for his WRX I don't know if the codes would be the same for my 2000 RS. Does anyone know of a list for us and if so how I can find it. I tried a search and came up empty handed (probably didn't type the right thing in the right place)
Wayne

Susie-
Don't worry about the CEL going off, as long as nobody does a reset the code will remain in memory in the ECU so the dealer will be able to find it. As I said previously you probably need a new cat and it may be marginal at this point so sometimes the 0420 will set and sometimes it will go away. Just take it to the dealer as per your appointment and it should get taken care of. Post back here afterward and let us know how it turned out.

Originally posted by War2.5RS My friend was telling me he printed out a complete list of CE codes for his WRX I don't know if the codes would be the same for my 2000 RS. Does anyone know of a list for us and if so how I can find it. I tried a search and came up empty handed (probably didn't type the right thing in the right place)
Wayne

They are the same. Look in herehttp://www.northursalia.com
Click on Tech Docs and informational pages. Scroll down and you'll find OBD II Trouble Diagnostic Codes

Last edited by North Ursalia; 10-16-2012 at 11:04 AM.
Reason: Fixed URL

Fixes tried: I have reset the ECU, only to have it come back. I have full exhaust, hks headers, and a manual boost controller set to 16psi. The exhaust/headers were installed a year ago. The MBC was installed a few months ago. The CEL showed up a little after the MBC was installed. Not making any assumptions, but would 16psi from an MBC be causing all of this? The connections are tight on the exhaust and egt.

[UPDATED FIX: Put 2.2k, 0.5 watt resistor ($0.99 from Radio Shack) in the EGT connector plug, and left the EGT in the exhaust. CEL eliminated, and no side effects. Thanks Mulder]

This code (P0456) actually indicates a high circuit input from the EGT rather than high temp. High circuit input in this case means high resistance and is usually caused by the sensor having gone bad and becoming electrically open (infinite resistance) or open wiring between the sensor and ECU.
High resistance from the EGT sensor corresponds to low EGTs, lower resistance indicates higher EGTs. The sensor is a thermistor which lowers its resistance with increasing temperature.
Most likely your EGT sensor has gone bad and caused this code to set. If you have a catless uppipe you can safely remove it, replace it with a bolt and install a 2.2K resistor in its place to eliminate the CEL.

In regards to CEL #P1507 - Idle control system malfunction (fail-safe), I've had this one pop up 3 times (I suspect my current, yet to be checked CEL is the same thing).

Now, I called my dealer and they told me that Subaru of Canada (this may apply to the States too, I don't know for sure) has issued a goodwill warranty repair on the computer to rectify this as they've got lots of complaints about this code popping up. My car is currently out of warranty, but they tell me SOC is picking up the tab on this one.